An airplane's airframe and engines produce varying amounts of audible noise during takeoff and landing. For example, an aircraft's engines typically operate at or near maximum thrust as the aircraft departs from an airport, and lower thrust as the aircraft approaches an airport. Some aircraft engine noise can be partially suppressed at the engine nacelle inlet and the exhaust nozzle by noise absorbing liners. These liners can absorb acoustic energy by canceling acoustic reflected waves and/or converting acoustic energy into heat, and typically consist of a porous skin supported by an open-cell matrix. The open-cell matrix provides separation between the porous skin and a non-perforated backskin. Some have postulated that the porous skin, open cells, and non-perforated backskin of such liners combine to form a plurality of Helmholtz resonators that resonate in response to certain sound frequencies or certain bands of frequencies, and cancel sound waves reflected between the porous face skin and non-perforated backskin and/or subsequently convert sound energy to heat (via elastic or mechanical hysteresis caused by the resonant response of air within the resonator cavities and of the liner components), and thereby effectively absorb or dissipate at least a portion of generated engine noise.
FIG. 1 shows a cross-section of a typical double-degree-of-freedom resonator cavity 10 of a prior art acoustic liner for modern aircraft gas turbine engines. A typical acoustic liner may include a plurality of such cavities 10 arranged in a compact side-by-side array. A perforated face skin 12, an imperforate back skin 14, and cell walls 16 bound the cavity 10. A honeycomb matrix, or another similar open-cell matrix, of a type well known in the art, may define the cell walls 16. In this double-degree-of-freedom arrangement, a perforated septum 18 divides the cell into a first chamber 20a proximate to the perforated face skin 12, and a second chamber 20b proximate to the imperforate back skin. A single-degree-of-freedom resonator cavity can be constructed substantially the same as shown in FIG. 1, but without the septum 18. The percentage open area (“POA”) of the face skin 12 and the POA of the septum 18 are selected to reduce the acoustic impedances of the face skin 12 and septum 18 in order to permit a desired degree of propagation of sound waves through the membranes 12, 18 at targeted frequencies. Sound waves enter the cell 10 by propagating through the perforated face skin 12 in a direction that is substantially perpendicular to the face skin 12 (see arrows in FIG. 1). The entering sound waves travel through the column of air contained within the cavity 10 in a direction that is substantially parallel to a principal longitudinal axis “y” of the cavity 10. Because sound waves enter the cavity 10 and propagate through air within cavity 10 along a substantially straight and unaltered path, such resonator cavities can be referred to as “non-folding” cavities.
Referring again to FIG. 1, entering sound waves propagate through air contained within the first chamber 20a, through the perforated septum 18, through air contained within the second chamber 20b, and are reflected by the imperforate back skin 14 (which has a relatively high acoustic impedance). The overall depths “d1,” “d2” of the two chambers 20a, 20b (as well as the thickness and POA of the face skin 12, the thickness and POA of the septum 18, the diameter/width of the cell 10, etc.) are selected such that the chambers 20a, 20b harmonically resonate in response to sound waves having one or more target frequencies and wavelengths. Configuring a resonator cavity to resonantly respond to a particular sound frequency sometimes is referred to as “tuning” the cavity. As shown in FIG. 1, the total depth “d3” of the cell is the sum of the depths “d1,” “d2” of the two chambers 20a, 20b and the thickness of the perforated septum 18. Depending on the available space (and possibly other engine design constraints), the total depth “d3” of the non-folding resonator cavities 10 of an acoustic liner often cannot exceed a maximum permissible depth “dmax” (i.e. d3≦dmax), as discussed below.
In practice, the limited maximum depth “dmax” of a conventional acoustic liner with non-folding resonator cavities can restrict the use of such liners to the absorption of relatively high-frequency, short-wavelength noise, such as noise generated by the rapid rotation of an engine's fan and compressor and turbine blades (i.e., frequencies from about 800 Hz to about 6000 Hz). In other words, for low frequency sound waves less than about 800 Hz, the total depth “d3” necessary to form a deep, large-volume, non-folding cavity 10 that can be tuned to harmonically resonate at such low frequencies may exceed the available depth “dmax”. Thus, though traditional non-folding acoustic liners may not be adaptable to absorb and dissipate relatively low frequency sound energy (i.e., less than about 800 Hz) like that commonly produced by an aircraft gas turbine engine's combustor (sometimes referred to as “core noise”).
Government regulators increasingly mandate aircraft engines with reduced noise signatures, and as a result, aircraft manufacturers, airline companies, and airport communities frequently demand such engines on aircraft. In order to achieve further reductions in modern aircraft gas turbine engine noise levels, especially during aircraft takeoffs and approaches, it is desirable to dissipate some of the low-frequency noise generated by an engine's combustor. Accordingly, there is a need for an aircraft gas turbine engine with acoustic treatment at or near the engine's combustor exhaust. More particularly, there is a need for an aircraft gas turbine engine with acoustic treatment at or near the engine's exhaust nozzle that is effective at dissipating some sound energy at frequencies less than about 800 Hz. In particular, there is a need for an acoustically treated hot nozzle center plug for a aircraft gas turbine engine that is capable of dissipating some combustor exhaust noise having one or more frequencies less than about 800 Hz.